This invention relates to fast optical shuttering and more particularly to a Q-switch in a laser.
Q-switching permits the generation of intense bursts of radiation from a laser by creating a highly regenerative feedback of energy from the laser gain element wherein energy is stored in the system then suddenly released. To switch from the low-Q build-up condition to output requires a Q-switch. Rotating mirrors, prisms, Pockels cells and saturable absorbers have all been employed as Q-switches. Due to practical difficulties with these devices, electro-optic and acousto-optic Q-switches have been proposed.
Acousto-optic (A-O) Q-switches are preferable to electro-optic switches because they are mechanically more simple and reliable. Q-switches previously described use interrupted continuous wave forms to control the high-Q oscillatory state. However, propagation of the acoustic wave through the material is relatively slow, and acousto-optic cells have not been generally employed where fast-acting Q-switches are required for short radiation bursts.
The inventor has helped describe a "Fast Acousto-Optic Lens Q-Switch" in the Journal of Applied Physics, Vol. 46, No. 8, p 3483, August 1975. This Q-switch makes use of the fact that the angle of diffraction of light from an acousto-optic diffraction cell is proportional to the frequency of the acoustic pulse in the cell. A fast chirp frequency causes incident light to be focused to a line as it would be by a cylindrical lens. A pulse with linearly varying frequency is termed generally a "chirped" signal. If the leading edge of the pulse contains lower frequency components, the signal is more specifically denoted an "inverted chirp."
An acoustic chirp signal is thus used to focus laser energy and acts as an acousto-optic lens. The focused beam is reflected back to its source and the undiffracted beam is used as the laser output.
The focal point of the diffracted beam is swept across an aperture at a speed greater than the velocity of the acoustic waves through the A-O cell. This permits a short high-Q time and results in a short on time. The rate of sweep across the aperture is proportional to the focal length of the diffracted beam. The frequency of the chirp signal controls the angle of diffraction and changing the frequency causes the sweeping action. The on time of the laser is thus inversely proportional to the bandwidth of the chirp signal. Using present A-O materials, a 20 nanosecond high-Q condition with the attendant laser pulse can be realized.
A discussion of laser arrangements using this type of Q-switch may be found in the Scott et al. application and the Journal paper reference above. A disadvantage of these previous arrangements is that there are two output beams, one from each mirror of the cavity. A further disadvantage of the previous arrangement is the optical path length. Longer optical paths require greater aligning and focusing accuracy.